In the brain, the neurotransmitter is present in the vesicles of pre-synaptic neurons. When the signal is given, the GABA contained in these vesicles is released. It then attaches to its receptor, located on the post-synaptic neuron. The receiver at the GABA then opens and allows chloride ions to enter this neuron (the post-synaptic one).
It is said that the pre- and post-synaptic neurons are synaptic. The GABA is involved in 30 to 40% of the brain's synapses.
As GABA is an inhibitory neurotransmitter, it is used in the treatment of epilepsy.